jlparsons
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| posted on 12/3/08 at 01:18 AM |
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Controlling electric reverse
Following the thread on electric reverse systems I was pondering the other day on the best way to control one rather than simply putting a switch on
the dash. I've worked out an idea, tell me what you think of this.
The car (BEC) will have a forward-back stickshift on the tunnel. You'd engage reverse by sticking the gearbox in neutral, putting your foot on
the brake pedal then pushing the shift lever to the left. You then activate the reversing motor with the brake pedal - release brake, motor pushes
car back. Push brake, motor cuts out. To get out of reverse you just slide the lever back to the right and you're ready to go.
The stick would normally be locked into the right (forwards) position. The electrical system would unlatch the shifter to allow it to move left and
select reverse only if your foot is on the brake, the gearbox is in neutral and the speedo is registering zero (or close enough). When in the left
position the shifter cannot be pushed forward or back to engage a gear. The latch will be failsafe in that no condition will prevent the gearstick
being moved back to the right, ie you can always get out of reverse.
I guess I like this idea as it controls a bolted-on reverse using pedals and gearstick much like a CEC, which I will appreciate if i use the car as
much as i'm hoping. Also I just like it when things are integrated and seamless, and of course if i'm honest i enjoy an engineering
problem more than i enjoy using the finished item. The mechanical bits are not difficult and don't present too much extra work given that I
probably will fabricate a stickshift anyway, and the electronic bits aren't too tricky. Also I don't think any bits involved would be
expensive.
On the other hand of course there's no actual difference in usability when compared to the button-on-the-dash and i'm not confident that
having got to that stage of the build i won't already have cashed in all my willpower. Then there's the obvious question - what would the
SVA man make of it?
What you think? Anyone done simlar? See any problems?
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matt_claydon
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| posted on 12/3/08 at 01:36 AM |
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I think it sounds like a great idea, but to my mind a little over-complicated with fail-safes / lockouts. (You can happily try to select reverse in
CECs whilst moving forwards).
I do like the idea of controlling the motor from the brake pedal, that's quite elegant. There is actually a distinct practical advantage here
too - you can keep both hands on the wheel.
I would have a dash switch or button on the tunnel to activate reverse, this would be wired in with the brake switch and neutral light so that the
motor comes on with the switch on, the engine in neutral and the brake pedal up. Any one action out of applying the brake, selecting a gear or
switching off the switch would cease reversing.
This setup is simple, purely electrical and uses components already present on the vehicle except for the master switch and relay.
Don't see how it would be a problem at SVA, they don't check anything to do with reverse gears.
[Edited on 12/3/08 by matt_claydon]
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jlparsons
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| posted on 12/3/08 at 01:48 AM |
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So you can engage the reverse at speed? That is good to know. I was working on the assumption that to do so would be at best expensive and at worst
dangerous.
I like your idea. All you'd need is a push switch and a reverse gear light, then selecting a gear or pushing in the clutch would deselect
reverse. A lot simpler...
I still kind of like the notion of the stickshift selecting reverse though. I'm not sure why, come to think of it. Perhaps because it feels
more like what you'd find on a production car? I'm sure that shouldn't make a difference! I'll think on it, your idea seems
a lot simpler and simple is usually better.
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nitram38
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| posted on 12/3/08 at 05:48 AM |
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Sounds too complicated to me and something more to go wrong.
I am using a solenoid type starter which will be wired through a toggle switch (off-on-spring). This will also have a relay controlled by the neutral
light for safety.
I will just put the bec in neutral, lift a doomsday cover.
Switching the toggle to the ON position will bring on the reverse light, in the spring position it will engage the solenoid starter.
To stop the car, all I have to do is release the spring position. This means it is quick to stop.
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ReMan
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| posted on 12/3/08 at 07:27 AM |
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Despite alll the failsafes and protections from putting it in reverse at the wrong time, which are good. You still end up with a system where, unless
you are willfully pressing the brake pedal, you have a car that will move or continue to move on it's own. ie the opposite of the accelerator
pedal where if you take your foot off you slow down, that does'nt seem right?
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jlparsons
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| posted on 12/3/08 at 08:16 AM |
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quote:
Originally posted by ReMan
Despite alll the failsafes and protections from putting it in reverse at the wrong time, which are good. You still end up with a system where, unless
you are willfully pressing the brake pedal, you have a car that will move or continue to move on it's own. ie the opposite of the accelerator
pedal where if you take your foot off you slow down, that does'nt seem right?
It's just like an automatic - once it's in gear, you take your foot off the brake and it crawls forward/back. It's odd the first
time you drive one but after ten minutes it's fine. The only safety issue is to ensure that anyone who sits in the drivers seat knows how
it's going to work before they use it, but even then you'd always instinctively jab for the brake pedal if the car's moving and you
don't want it to be.
On the other hand if you wanted a deadman function you could use a clutch switch and ensure the clutch has to be in for the reverse to work, I
pondered that but in the end decided it might be overkill.
[Edited on 12/3/08 by jlparsons]
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MikeRJ
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| posted on 12/3/08 at 12:28 PM |
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quote:
Originally posted by ReMan
Despite alll the failsafes and protections from putting it in reverse at the wrong time, which are good. You still end up with a system where, unless
you are willfully pressing the brake pedal, you have a car that will move or continue to move on it's own. ie the opposite of the accelerator
pedal where if you take your foot off you slow down, that does'nt seem right?
I agree, it doesn't sound ideal from this perspective. Some autos can creep at idle with drive selected, but it's not really the same
thing as a) it's very slow, b) there is no direct drive to the wheels so the actual torque available is very small.
Why not a simple pushbutton mounted on the steering wheel with an inhibitor connected to the neutral light and maybe also a shielded switch on the
dash (so you have to select neutral, set the reverse switch and then push the button to actually reverse).
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matt_claydon
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| posted on 12/3/08 at 12:31 PM |
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quote:
Originally posted by jlparsons
So you can engage the reverse at speed?
No, but you'd have to be pretty stupid to! Like I said, there's nothing to stop you putting a manual gearbox (or an auto for that matter)
into reverse when you are moving forward, and yet people don't do it all the time and mash their 'boxes!
I think just using a spring-loaded toggle has a distinct flaw of having to steer with one hand which this idea overcomes. Doing a tight reverse-park
or reverse round a corner with only one hand to steer must be quite awkward?
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bartonp
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| posted on 12/3/08 at 12:44 PM |
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Why not try to mimic a 'real' reverse gear?
Reverse selector sw interlocked with neutral. Clutch up starts motor and throttle position controls the motor speed....
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jlparsons
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| posted on 12/3/08 at 01:43 PM |
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quote:
Originally posted by bartonp
Why not try to mimic a 'real' reverse gear?
Reverse selector sw interlocked with neutral. Clutch up starts motor and throttle position controls the motor speed....
I did think of that one, though the deadman position is still the same - going backwards. The opposite may actually be better, ie the reverse
won't engage without the clutch pushed in, therefore the deadman position is power-off. This is counter-intuitive though and I'm not
convinced it's necessary.
With adequate safeguards to prevent engaging reverse without meaning it I'm convinced that two hands on the wheel and foot covering the brake is
the safest control configuration to be using reverse, at least that's what I've concluded for the moment. I'm sure that's
much safer than one hand on the wheel and one hand on a switch.
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jlparsons
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| posted on 12/3/08 at 01:45 PM |
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quote:
Originally posted by bartonp
...and throttle position controls the motor speed....
Love to - but most units seem to be on/off. That's a whole project in itself, unless there's an easy way?
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chunkytfg
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| posted on 12/3/08 at 01:50 PM |
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What about trying to engage the reverse with the clutch.
Take a flywheel type thing on the output shaft/diff depending on where you want to put it and then have the motor engaged against the side of the
wheel at 90deg but with it able to pivot off the wheel actuated by the clutch lever. This esentially allows you to slip the clutch to control the
speed.
Just an idea though. No idea if you could get it to work.
Bit like this
You would need a way of making sure the motor slips and also a way of locking the motor away from the flywheel while in normal use
[Edited on 12/3/08 by chunkytfg]
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chunkytfg
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| posted on 12/3/08 at 02:02 PM |
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And yes i am fully aware of a lack of ability to draw using MS Paint!!!
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jlparsons
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| posted on 12/3/08 at 02:50 PM |
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Looks like a complicated way of controlling output, and it would be difficult to make it reliable. Rather than go to the effort of effectively
clutching the electric motor you could just divert the output from the throttle sensor to a stepper to vary the power to the motor when in reverse.
But would either be worth it? Being able to trigger reverse with the pedals whilst having both hands on the wheel I think is worth a bit of work, but
to control the output level would be many times the effort and I don't think the benefit would justify it.
You look fit in them pigtails by the way.
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matt_claydon
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| posted on 12/3/08 at 03:56 PM |
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quote:
Originally posted by Bob C
I also don't really like the thought that if I let go of everything the car will shoot off backwards.
I don't really see why so many people have a problem with this? It's exactly what would happen if you released all controls in a normal
car, both auto and manual.
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jlparsons
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| posted on 12/3/08 at 05:55 PM |
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Well, I think i've decided a couple of things -
First i definitly like the idea of running the motor off the brake pedal. It's going to be a lot safer having two hands on the wheel and
equally imortant it's also easier to look around you 180 degrees without having to have one hand on a switch. The fact the deadman position is
with the power on isn't a bad thing provided the safeguards make sure it can only be activated by a live man, as with an automatic tintop (or
even a manual for that matter). The fact that you have to have your foot covering the brake at all times when reversing is also a good thing for
safety.
Second, I think i do like the idea of having reverse engaged by pushing the gearshifter to the left. It feels a more elegant and intuitive solution
than to have a seperate button; it makes sense to have forward and reverse gears selected by the same control and ensures only one can be selected at
a time. Also it'll be harder to engage reverse accidentally as the gearstick will be in a different position from neutral. This reduces to near
zero the posibility of thinking the car is in neutral and taking your foot off the brake, only to find it's actually in reverse and suddenly
you're shooting backwards. Also I like the stickshift idea for the fact that you have both electrical and mechancal safeguards - mechanically
moving the stick left is the only way to put power to the reverse, and because the stick can always be pulled back to the right, no electrical failure
can leave you unable to deselect reverse.
I'm thinking this has to be tried! I'm adding to my (long) list...
[Edited on 12/3/08 by jlparsons]
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nitram38
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| posted on 12/3/08 at 07:14 PM |
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quote:
Originally posted by matt_claydon
quote:
Originally posted by jlparsons
So you can engage the reverse at speed?
No, but you'd have to be pretty stupid to! Like I said, there's nothing to stop you putting a manual gearbox (or an auto for that matter)
into reverse when you are moving forward, and yet people don't do it all the time and mash their 'boxes!
I think just using a spring-loaded toggle has a distinct flaw of having to steer with one hand which this idea overcomes. Doing a tight reverse-park
or reverse round a corner with only one hand to steer must be quite awkward?
When was the last time you reversed around a corner?
Also regarding taking your hand off the wheel, it is regarded as a safe way by driving instructors (in fact the only time) to enable you to turn your
head enough to get a clear view behind you.
Keeping both hands on the wheel could compromise your view.
If your reverse is geared correctly, the car should move very slowly. Applying the foot brake slightly will not harm your starter motor. In fact a
free spinning starter uses more current than a loaded one.
The technical reason for this is to do with an extremely low resistance of a starter motor which draws large currents.
When you load a starter motor, there is a back emf created which efectively raises your "resistance" and therefore cuts down on the
supply current.
That is why you should not connect a starter without a load. Apart from the thing spinning or lurching (even out of a vise) the rise in current is
enough to burn it out.
[Edited on 12/3/2008 by nitram38]
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matt_claydon
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| posted on 12/3/08 at 07:51 PM |
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quote:
Originally posted by nitram38
When was the last time you reversed around a corner?
Every time I have to park in a car park or do a three-point turn. Depends on your rack ratio I guess but in both my Indy and tintop I need to turn the
wheel more than can be done with one hand to make the manouvre.
quote:
Also regarding taking your hand off the wheel, it is regarded as a safe way by driving instructors (in fact the only time) to enable you to turn your
head enough to get a clear view behind you.
Keeping both hands on the wheel could compromise your view.
I'm not saying you HAVE to keep both hands on the wheel! Just that if you need to turn it further than you can with one movement of one hand
then it is easier and safer if you can use the other hand than have to let go of the wheel and re-grip it elsewhere.
quote:
If your reverse is geared correctly, the car should move very slowly. Applying the foot brake slightly will not harm your starter motor. In fact a
free spinning starter uses more current than a loaded one.
The technical reason for this is to do with an extremely low resistance of a starter motor which draws large currents.
When you load a starter motor, there is a back emf created which efectively raises your "resistance" and therefore cuts down on the
supply current.
I think you've got that backwards, the greatest back-emf is created when the motor is turning fast, an unloaded motor uses less current than a
loaded one.
It is topsy turvy to think that a motor under load is consuming less power (and therefore current) than an unloaded one. For the same reason a motor
produces its maximum torque when it is stalled.
From: http://en.wikipedia.org/wiki/Brushed_DC_Electric_Motor
"As an unloaded DC motor spins, it generates a backwards-flowing electromotive force that resists the current being applied to the motor. The
current flow through the motor drops as the rotational speed increases, and a free-spinning motor has very little current flow. It is only when a load
is applied to the motor that slows the rotor that the current draw through the motor increases."
There is no right and wrong way here, it's whatever you feel is best. For me I feel it would be most ergonomic to control motion with my feet
and direction with both my hands.
[Edited on 12/3/08 by matt_claydon]
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MikeRJ
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| posted on 12/3/08 at 07:56 PM |
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quote:
Originally posted by nitram38
That is why you should not connect a starter without a load. Apart from the thing spinning or lurching (even out of a vise) the rise in current is
enough to burn it out.
The danger with powering a series wound starter motor with no load is not too much current burning it out (current decreases with increasing RPM), but
the fact that series motors are not self regulating so an unloaded motor can easily run past it's safe maximum RPM.
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jlparsons
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| posted on 12/3/08 at 08:45 PM |
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Regarding the debate about two hands on the wheel when reversing, i think the main point is that as your foot is controlling the brake and the motor
by default you're free to do anything you like with your two hands, not necessarily have both on the wheel. The fact that you don't need
to have one hand on the wheel and the other on a switch means you are free to turn around and look straight behind you - try doing that with one hand
on the dash and the other on the wheel! I tend to leave one hand onthe wheel and put one hand on the passenger seat or the sill to the right,
depending on which way I'm turning, then I can see around behind me properly. If I did that in a regular switched electric reverse, I
wouldn't be able to turn on the motor to actually move...
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Bob C
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| posted on 12/3/08 at 10:01 PM |
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argh - my previous post had appeared twice, so I deleted one & both disappeared. A lot of old starters were series wound, & a lot of new ones
are permanent magnet commutator motors, often with an epicyclic reduction gearbox. They have very different behaviors. My issue is that I didn't
want a switch, I wanted to be able to control the speed/torque via a pot. I can see if you just want to switch it, the brake switch is all ready made
(though it's likely to 'chatter' if you're not careful) And I still don't like the fact that it will just set off
backwards if you select reverse....
For a pot, I can't use the throttle or the engine will race. The clutch is in the frame, though it's all counter intuitive... I thought
about pulling the throttle up to control reverse - but I don't like that either - it would be hard to find the brake in a hurry. I guess I need
to think about this while I'm sat in the car......
Bob
[Edited on 12/3/08 by Bob C]
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C10CoryM
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| posted on 13/3/08 at 02:36 AM |
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How about something to the effect of:
1: whatever lock out you want to ensure you are in N
2: press or release clutch pedal to engage motor.
For the lock out maybe a electric solenoid to keep you from moving the shifter to the left unless you are in N. Or just a covered switch on the
dash.
This way you only need one switch on the clutch pedal and one safety switch to activate it. Both hands on wheel and you have your right foot primed
on the brake so you can quickly stop quickly. I imagine reverse is a little jerky with electric motors so the ability to bump and stop an inch at a
time might be nice.
Wonder if you could get a slow ramping switch to reduce the instant on effect w/o damaging the starter?
"Our watchword evermore shall be: The Maple Leaf Forever!"
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jlparsons
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| posted on 13/3/08 at 09:25 AM |
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quote:
Originally posted by C10CoryM
How about something to the effect of:
1: whatever lock out you want to ensure you are in N
2: press or release clutch pedal to engage motor.
For the lock out maybe a electric solenoid to keep you from moving the shifter to the left unless you are in N. Or just a covered switch on the
dash.
This way you only need one switch on the clutch pedal and one safety switch to activate it. Both hands on wheel and you have your right foot primed
on the brake so you can quickly stop quickly. I imagine reverse is a little jerky with electric motors so the ability to bump and stop an inch at a
time might be nice.
Wonder if you could get a slow ramping switch to reduce the instant on effect w/o damaging the starter?
I did think about that, but I concluded that if you're going to have to cover a pedal, it's better for it to be the brake. If
you're using the clutch to control it you're going to have to cover the brake anyway. On the other hand, using a clutch switch may be
easier to control, and given that it'd be so easy and quick to switch between the two it definitly warrants trying in case it's better in
use.
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approval, terms and conditions apply. Apply only to affected area. For recreational use only. All models over 18 years of age. No user-serviceable
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jlparsons
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| posted on 13/3/08 at 01:22 PM |
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quote:
Originally posted by jlparsons
quote:
Originally posted by C10CoryM
How about something to the effect of:
1: whatever lock out you want to ensure you are in N
2: press or release clutch pedal to engage motor.
For the lock out maybe a electric solenoid to keep you from moving the shifter to the left unless you are in N. Or just a covered switch on the
dash.
This way you only need one switch on the clutch pedal and one safety switch to activate it. Both hands on wheel and you have your right foot primed
on the brake so you can quickly stop quickly. I imagine reverse is a little jerky with electric motors so the ability to bump and stop an inch at a
time might be nice.
Wonder if you could get a slow ramping switch to reduce the instant on effect w/o damaging the starter?
I did think about that, but I concluded that if you're going to have to cover a pedal, it's better for it to be the brake. If
you're using the clutch to control it you're going to have to cover the brake anyway. On the other hand, using a clutch switch may be
easier to control, and given that it'd be so easy and quick to switch between the two it definitly warrants trying in case it's better in
use.
Actually, the satefy issue with using the clutch pedal is that you're also engaging the mechanical clutch as well - if by some fault you ended
up with the car in first when you thought you were in reverse, suddenly you're going the wrong way. q If you leave the clutch out of it you
can't engage another gear by mistake.
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